1. Field of the Invention
The present invention relates to a Digital Beam Forming (DBF) radar apparatus having a receiving antenna that includes a plurality of antenna elements, and that forms antenna beams digitally.
2. Description of the Related Art
In recent years, DBF type radar apparatuses provided with array antennae that include a plurality of antenna elements, and that form antenna beams digitally with the aid of a signal processing unit, have come into focus.
Each of the antenna elements in the array antenna is connected to a radio frequency (RF) amplifier, a mixer, a filter, and an A/D converter. A digital beam forming processor takes in digital signals that are output by each A/D converter, and carries out a digital beam forming process.
Japanese Patent Laid-Open Publication Nos. H11-160423 H11-064485 disclose DBF type radar apparatuses in which the number of expensive high-frequency analog devices that are attached to each antenna element are limited, as well as maintaining compactness and simplicity of the apparatus.
Japanese Patent Laid-Open-Publication No. H09-162626 discloses a monopulse radar apparatus having a flat, two-circuit array antenna including rows of antenna elements. Odd and even numbered rows of the antenna elements are arranged in an arch-shape to form teeth of a comb. With such structure, thereby enhancing antenna gain. Moreover, by maintaining a gap of less than λ between the phase centers of the array antenna, the increase in ambiguity due to phase aliasing at the time of phase difference detection is prevented in the monopulse radar apparatus, regardless of the increase in the number of antenna elements.
Many examples of radar apparatuses that use a phase comparison monopulse system are disclosed among conventional analog process type radar apparatuses. For example, Japanese Patent Laid-Open Publication No. H11-281729 discloses a radar apparatus that perform target search over a wide range, and an identification of multiple targets. Such radar apparatuses make use of amplitude change of reception signals generated from switching of a transmission beam, using a plurality of transmitting antennae.
The radar apparatuses disclosed in Japanese Patent Laid-Open Publication Nos. H11-160423 H11-064485 are obtained by eliminating some of the parts of the conventional DBF type radar apparatus, with a view to reducing production cost as well as making the product compact and simple. However, the advantages of the conventional DBF type radar apparatus cannot be guaranteed in these radar apparatuses.
Moreover, in these radar apparatuses, the objective was to limit the number of high frequency analog devices, and hence, the antenna system was not given much consideration. However, in many cases, the number of antenna elements, rather than the number of high frequency analog devices, presents problems such as constraints in mounting. For example, the number of antenna elements needs to be reduced when there are space constraints on the mounting platform.
In the radar apparatus disclosed in Japanese Patent Laid-Open Publication No. H09-162626, the antenna elements in the array antennae need to be coupled with one another (in a comb tooth like fashion) in each array antenna. Considering the size of the antenna element itself (usually about λ/2), and the need to avoid the influence of coupling between the antenna elements, it is difficult to narrow the spacing (for example, make it less than λ) between the antenna elements of the array antenna. Consequently, in the array antennae provided on such a radar apparatus, although the spacing between the phase centers of the two array antennae can be set below λ, the spacing between the antenna elements of the array antenna itself may be more than λ. Thus, it is impossible to effectively curb the range of ambiguity in the target direction due to the antenna pattern of the array antenna itself.
Further, in the radar apparatus disclosed in Japanese Patent Laid-Open Publication No. H09-162626, one antenna element is associated with either of the array antennae, and the concept of DBF process, in which the output of the antenna elements is repeatedly used between different array antennae, is absent. Thus, it is difficult to assemble and arrange the antenna elements of the array antennae, and the number of antenna elements has to be reduced when there are space constraints on the mounting platform.
Moreover, the degree of freedom required in controlling the beam width and the beam bearing of each array antenna is almost absent in the radar apparatus disclosed in Japanese Patent Laid-Open Publication No. H09-162626. Thus, it is difficult to carry out monopulse processes involving a high degree of freedom, such as limiting the search area or substantially simultaneously searching multiple search areas.
The radar apparatus disclosed in Japanese Patent Laid-Open Publication No. H11-281729 carries out the phase comparison monopulse process by switching transmission beams using a plurality of transmitting antennae. Because considerable space is required for mounting the transmitting antennae, it is impossible to implement the system when there are space constraints on the platform used for mounting the transmitting antennae.